The combining of cellulose-based materials to plastics was originally developed over 25 years ago by extrusion machinery manufacturer ICMA San Giorgio(1) and used by G.O.R. Applicazioni Speciali SpA to make door panels for FIAT cars. The materials for this process were pre-mixed and cram-fed.
Specialist machines have recently been developed in the composite market to produce window and door profiles, as well as decking boards, using hardwood and softwood flour. Generally this equipment is based on traditional plastics manufacturing technologies including extrusion and injection moulding. The plastics used include polypropylene (PP), polyethylene (PE) and poly-vinyl-chloride (PVC) and the fillers used include wood flour, flax, jute and other cellulose-based fibre fillers. The more cellulose-based material that is added to the plastic, often the lower the price and, often, the higher the stiffness of the wood-plastic “raw” material. The composite products made from these wood-plastics can generally be nailed, painted and otherwise treated as wood whilst potentially retaining many of the benefits of plastics in the areas of fungal and corrosion resistance.
For some, addition of wood flour has drawbacks in that, compared conventional inorganic fillers for plastics, it is low bulk density and often needs significant pre-drying before or during compounding, which can result in low production rates and high costs. The powdery consistency of such fillers not only results in a messy operation, and may pose potential health risks to those manning the processing. Wood flour (and wood fibre) also tend to cause blocking in addition port or hoppers, bridging or agglomeration due to the material packing together and can be more difficult to convey and feed into an extruder compared to conventional plastic fillers, the inlet of which is typically small relative to the low bulk density of these materials.
A number of commercial enterprises have recognised some of these problems and have developed, for sale, pre-pelletised wood flour for convenient feeding into extruders(2). Other have patented processes or concepts around production of pellets containing wood flour and thermoplastic material for further processing such as extrusion(3). Some have compatibilisers or additives to give enhanced properties(4), with most still focused on wood flour.
Wood flour is a finely ground wood cellulose. When the particle size is above 20 mesh or below 850 microns, the product generally is considered to be wood flour. Mesh size is the measurement of number of openings in a screen per linear inch. The collected wood flour from various sources (sawdust, planar shavings, sanding dust and scraps) are hammer milled to form very fine powder, classified by the standard mesh size that it can pass through. Most wood-filled thermoplastic manufacturers specify flour in the 30-80 mesh range. Bulk density of wood flour is relatively higher compared to wood fibres. The moisture content of wood has a significant effect on the processing and final composite product quality. Pre-drying the wood flour to less than 1% moisture content is usually desirable. Wood flour with less moisture content is less likely to burn during compounding with thermoplastics. The particle size for other natural fibres/fillers such as pine needles, maple, oak, bamboo dust, jute and coir may vary from 10-80 mesh. (From: Thermoplastic Composites—a New Business Avenue, M Suresh Babu, Sangeeta Baksi, G. Srikant & Soumitra Biswas (www.tifac.org.in/news/acthermocomp.htm).
Commercial wood flour often comes in mesh sizes of 20 to 100, but most thermoplastic applications are in the 30 or 40 mesh to 80 mesh range. Wood “fibre” is, strictly speaking, not the same as wood flour though, confusingly, the term “fibre” is used interchangeably for flours. Wood fibre, as opposed to wood flour, will typically be longer fibre-like rather than particle-like (flour) and may typically have average lengths of >0.85 mm, and more usually >1 mm, and perhaps in the 2-3 mm range. Some powder (fines) may be present in the fibre products, but they are usually minor components and average lengths are thus greater in fibre rather than flour. In addition, often, fibre is more entangled or fluffy, of lower bulk density, and difficult to handle. The aspect ratio (length÷diameter) of, for example, wood fibres may be >10:1 and may typically be, for example, of the order of 20:1, 25:1 or 40:1, 50:1, 70:1 or more, while “flour” may generally have aspect ratios of 1:1 to 4:1 or 5:1, and typically less than 8:1 or 10:1. Some wood flour is pulverized flour as fine as 200 mesh. There may be exceptions, but flour is generally a powdery product and fibre a longer fibrous-like material. Wood flour adds some stiffness to plastics but can reduce strength and/or impact strength. However, the longer, higher aspect ratio, wood fibres contribute more to either or both stiffness and strength, compared to wood flour, while being lighter than many synthetic fibres. However, wood fibres are more difficult to blend and bind into the plastic-composites. The performance advantages of using wood fibres instead of wood flour have been recognised(5) but attempts to produce special equipment to handle wood fibre to compound with a plastic have met with limited success. Special fibre feeders or ‘stuffers’ or ‘crammers’ are available but are often expensive and not reliable over a wide range of metering for end compositions since, for example, some applications may require <10 wt % fibres while others may require more than this and indeed >50 wt %. Mechanical properties including creep resistance have the potential to be improved by use of longer fibres compared to flours or powder fillers.
The introduction of low bulk density natural or wood fibres into extruders or injection moulders or other plastics processing machinery, in particular in a metered or measured way, which is important to achieve desired fibre volume fractions in compositions, has a number of difficulties. Thus, it is not staightforward, due to the inherent high volume/low mass nature of such fibres, the lack of free flowing characteristics in such fibres, and the fibre bundling or entanglements, to achieve controlled feeding directly into port holes or orifices of plastics processing machinery. Although some fibre-feeders exist they are either expensive and/or unreliable or inaccurate in metering uniformly over a wide range of fibre feed ratios with wood and other natural fibres. In addition, it is necessary to pre-dry a high volume-low mass of fibre before such feeding/use since such fibres are hygroscopic and retain, or reabsorb, high levels of water, which is usually undesired and required to be substantially removed prior to the plastic processing. (See, for a review of fibre feeders, John Winski, “Feeding Solutions for Wood Plastics Applications”, The 6th International Conference on Woodfiber—Plastic Composites, p 137-148). Thus, the processing and handling issues mentioned above associated with wood flour are much worse when one wishes to consider use of wood fibre. Hence, if a convenient and low cost method for the manufacture of wood fibre pellets existed this would be a breakthrough for wood fibre utilisation in plastics. Such products with suitable performance and/or convenient, cost-effective, methods for their manufacture, are not well known or established. Feeding or metered additions of pellets into extruders etc is much more convenient.
Medium density fibreboard (MDF) uses a high temperature thermomechanical pulp fibre to produce large panels for a variety of applications, such as furniture or internal mouldings. Due to the commercial scale of these operations and the use of heat to soften the fibres, MDF fibre is a low cost form of wood fibre. Additionally, it has an aspect ratio to allow reinforcing of composites (for example radiata pine approx 2.5 mm×30 μm). In the MDF process a thermosetting resin, typically urea-formaldehyde (UF) or related formaldehyde crosslinking resins, or other resins such as isocyanate resins, is added to the fibre, with the fibre in a wet state, while it is exiting the refiner in the blowline. This gives extremely high surface coverage of the fibre at low resin loadings(6). The MDF process would not be usually associated with producing a thermoplastic precursor for further processing in extrusion or other thermoplastic operations. Ordinarily it produces a sheet product which is fibre-rich and bound with a rigid, cured, thermoset resin which is not readily thermoplastically processable or reprocessable or easily usable or useful in thermoplastic processes.
Plastic and wood, or plastic and natural fibres, do not mix easily, although some polymers are more compatible than others are. PVC, which is polar like wood, reportedly bonds to, or interacts well with, the filler or fibre without special alloying or coupling agents, whereas polyolefins (polypropylene and polyethylene) do not adhere to wood as well as PVC, and so such wood-plastics require modification to get the best level of performance from the filler or fibre in the plastic. One example of the current state of technology is to add a coupling agent, often a maleated polyolefin for polyolefin based composites, into the extruder and mixing within the extruder. The prior art contains numerous suggestions regarding polymer fibre composites. Gaylord, U.S. Pat. Nos. 3,765,934, 3,869,432, 3,894,975, 3,900,685, 3,958,069 and Casper et al., U.S. Pat. No. 4,051,214 teach a bulk polymerization that occurs in situ between styrene and maleic anhydride monomer combined with wood fibre to prepare a polymer fibre composite. Segaud, U.S. Pat. No. 4,528,303 teaches a composite composition containing a polymer, a reinforcing mineral filler and a coupling agent that increases the compatibility between the filler and the polymer. The prior art also recognizes modifying the fibre component of a composite. Hamed, U.S. Pat. No. 3,943,079 teaches subjecting unregenerated discontinuous cellulose fibre to a shearing force in shear mixers, resulting in mixing of a polymer and a lubricant material with the fibre. Such processing improves fibre separation and prevents agglomeration. Similarly, Coran et al., U.S. Pat. No. 4,414,267 teaches a treatment of fibre with an aqueous dispersion of a vinyl chloride polymer and a plasticizer, the resulting fibres contain a coating of polyvinyl chloride and plasticizer and can be incorporated into the polymer matrix with reduced mixing energy. Beshay, U.S. Pat. Nos. 4,717,742 and 4,820,749 teach a composite material containing a cellulose having grafted silane groups. Raj et al., U.S. Pat. No. 5,120,776 teach cellulosic fibres pre-treated with maleic or phthalic anhydride to improve the bonding and dispersibility of the fibre in the polymer matrix. Raj et al. teach a high density polyethylene chemical treated pulp composite. Hon, U.S. Pat. No. 5,288,772 discloses fibre reinforced thermoplastic made with a moisture pre-treated cellulosic material such as discarded newspapers having a lignant content. Kokta et al., “Composites of Poly(Vinyl Chloride) and Wood Fibres. Part II. Effect of Chemical Treatment”, Polymer Composites, April 1990, Volume 11, No. 2, teach a variety of cellulose treatments. The treatments include latex coating, grafting with vinyl monomers, grafting with acids or anhydrides, grafting with coupling agents such as maleic anhydride, abietic acid (See also Kokta, U.K. Application No. 2,192,397). Beshay, U.S. Pat. No. 5,153,241 teaches composite materials including a modified cellulose. The cellulose is modified with an organo titanium coupling agent which reacts with and reinforces the polymer phase. Maldas and Kokta, “Surface modification of wood fibres using maleic anhydride and isocyanate as coating components and their performance in polystyrene composites”, Journal Adhesion Science Technology, 1991, pp. 1-14 show polystyrene flour composites containing a maleic anhydride modified wood flour. A number of publications including Kokta et al., “Composites of Polyvinyl Chloride-Wood Fibres. III: Effect of Silane as Coupling Agent”, Journal of Vinyl Technology, Vol. 12, No. 3, September 1990, pp. 142-153 disclose modified polymer (other references disclosed modified fibre) in highly plasticized thermoplastic composites. Additionally, Chahyadi et al., “Wood Flour/Polypropylene Composites: Influence of Maleated Polypropylene and Process and Composition Variables on Mechanical Properties”, International Journal Polymeric Materials, Volume 15, 1991, pp. 2144 discuss polypropylene composites having polymer backbone modified with maleic anhydride.
Although many publications refer to wood fibres, in reality they are essentially wood flour or powders, or flakes, or saw-dust etc rather than fibres with a reasonable aspect ratio (10:1 or 20:1 or more, etc as described above), for example typically of average length 1 mm or more. Flour and related materials are easier to handle and process and feed into typical plastics machinery, whereas the longer fibres tend to be entangled and fluffed and much more difficult to feed into plastics machinery in a metered way.
Accordingly, a substantial need exists for improved processes to introduce wood or other natural fibres, for example with an aspect ratio greater than 10:1 or 20:1 or with an average fibre length of, say, 0.8 mm or 0.9 mm or 1 mm or more (as opposed to wood flour or powders or flakes) into plastics processing machinery, and, also, for compatibilising formulations or materials for combining thermoplastic polymer(s) and wood or natural fibre(s).
In other prior art, Sears et al (9) describes use of fibres with an alpha cellulose content purity >80% indicative of pulps which are usually kraft or chemically pulped and not ordinarily available or manufactured by an MDF type process ie via mechanical or thermomechanical pulping methods such as used in the present invention. Fibre pellets with wet pulp cellulosic fibres can be manufactured in other ways such as use of granluated pulps impregnated with binders, which will often reduce fibre lengths and aspect ratios and result in a product which is similar to the use of wood flour, or via use of mixers in which fibres and aqueous, dissolved, binders are premixed (eg in Hobart or similar mixer) in a water medium, and then pellitised wet, via special pellet mills (eg Kahl Pellet Mill). Such methods are described by Sears et al (9). These approaches will not be as convenient as the present invention in that significant drying and densification of the wet impregnated pulps is required prior to eventual introduction to plastics and the process or products are unlikely to be as cost-effective as an MDF manufacturing approach, and the fibre products produced therein. In addition, the use of blowline or related processes which use moving air or steam carried fibre streams and polymer solutions or dispersions applied therein, coat the fibres with a well or highly dispersed polymer coverage on the fibre surface in highly efficient manner with efficient usage of polymer. Furthermore the use of the MDF and related processes are well suited to low cost continuous manufacturing processes.